This invention relates to an image display apparatus, and specifically to an image display apparatus having one or more thermochromic images mounted on a heatable substrate so that the visible images on the substrate change as the temperature of the substrate is changed.
Signs and visual displays are often used to gain attention. Signs are used to warn of danger or to call attention (e.g., arrows or "PAY HERE"). In advertising, signs are used to draw attention to a product or store or special event. A typical sign is two-dimensional and contains no movement or changing features. Mechanically movable signs and images are known (e.g., changeable billboards), but are often very complex and require substantial extra space for the mechanical components, (e.g., motors, shutters, linkages, etc.). Other means to simulate or show motion or changing scenes in signs include varying the densities or colors of light on the sign, motion picture technologies (e.g., a backlit projector box) or video systems. Such arrangements are relatively expensive and require additional space in the signage for components necessary to achieve the desired and hopefully eye-catching display.
In many contexts, and particularly in the advertising context, space concerns are a premium. A point of purchase advertisement (advertising within a retail outlet directly to the consumer at the time of purchase) must compete with all other products and displays in the surrounding area for catching the consumer's eye and attention. Advertising space in this setting is extremely limited and often quite expensive. While the techniques mentioned above have been utilized to create active visual displays and signs, a more efficient and reliable arrangement is desired. In addition to being compact in its use of space, it is of course desirable that an active visual display be energy efficient and simple in operation, with very low maintenance concerns.
In Kito et al., U.S. Pat. No. 4,554,565, a method of producing a reversible thermochromic display is set forth. Kito et al., U.S. Pat. No. 4,554,565 is incorporated by reference herein. The Kito et al. patent is directed to method for producing a reversible thermochromic display composed of two or more overlapping layers of images formed on a support. At least one of the layers provides a reversible thermochromic image, and the colored images on the respective layers thus have different visual densities depending on the temperature of the support. The reversible thermochromic image layer changes color at a temperature lower or higher than a predetermined point and provides a color which is more intense and more easily discernable than the pale color of the other images. The Kito et al. patent fails to disclose any particular heater structure for heating the support on which the image layers are placed.
The Kito et al. patent advocates a thermochromic image layered approach to forming an active visual display. While the reversible thermochromic material has a light transmissive state and a non-light transmissive state, it does suffer from the problem of not clearing to full transparency in its light transmissive state. It clears to a slightly whitish milky state, not fully transparent. Accordingly, applying layers of thermochromic inks over one another builds up this non-transparency feature when the layers are successively cleared, which significantly impairs the clarity of any underlying images.
Reversible thermochromic materials are known (see, e.g., U.S. Pat. Nos. 4,028,118 and 4,720,301, incorporated by reference herein) and printing inks and paints prepared from such materials have been applied to various articles to construct temperature indicators, ornamental objects and toys that reversibly change or develop color at well defined temperatures. For example, Hippely et al., U.S. Pat. No. 4,917,643, shows a toy vehicle painted with a thermochromic material which changes color as the temperature of the vehicle is varied. Shibahashi et al., U.S. Pat. No. 4,920,991, sets forth a thermochromic artificial fingernail which is either formed from or coated with a quasi-reversible material which is changeable in color and responds to temperature changes (with very high hysteresis effects), and therefore exhibit various external visual appearances. This reference notes that the thermochromic material may be formed not only as a layer which changes in color equally over its entire surface, but as a layer having thermochromic patterns so that the change of color is attained with some variety. It is suggested in Shibahashi et al. that the thermochromic material be formed as a layer having a plurality of thermochromic areas, which differ from one another in thermochromic response to temperature, and that such areas may be formed side-by-side to form patterns and may even include non-thermochromic areas. Shimizu et al., U.S. Pat. No. 4,560,604, discloses a method of flocking a shaped material with a thermochromic type material, wherein the shaped material can be used for making stuffed dolls, toy animals, etc. for infants to enjoy the color change in a bath. Reversible thermochromic materials have also been used for hidden messages on coffee cups, where the message appears on the side of the cup when the cup is filled with a hot liquid (or alternatively, with a cold liquid). The color conversion properties of these thermochromic materials may be generally linear such as the "Regular" type thermochromics available from Matsui International Co., Inc., of El Segundo, Calif., or non-linear such as the "History" type thermochromics available from Matsui International Co., Inc.
Another thermochromic material is a cholesteric liquid crystal, which is a "reversible" thermochromic that has a defined temperature range where the liquid crystal is optically active and scatters light in the visible range. The use of electrically activated liquid crystals in displays is found on the familiar numerical displays of watches and pocket calculators. Chiral nematic color changing liquid crystals are used in digital thermometers. These thermally activated liquid crystal materials are also used in battery testers and other temperature indicators, as well as various novelty products.
A further reversible thermochromic material is commonly referred to as "mercury salt". This material also has the characteristics of changing from one color to another (e.g., red to black) in response to changes in temperature, and the original color of this type of thermochromic material is restored by reversing the temperature change, with little hysteresis.
With respect to active sign-type displays using thermochromic materials, Three Tec Davis, Inc., of Tokyo, Japan, has made a display using a printed carbon heater with printed silver buss bars on a Mylar or thin di-electric film to provide a heat source using Joulean heating. A non-thermochromic silk-screened color message is placed on the film and covered by a high-temperature black thermochromic "shutter" which clears at high temperature to reveal the message thereunder. Placed on top of this high-temperature thermochromic black "shutter" are two liquid crystal messages. The liquid crystal messages are activated at two different temperature ranges, both which fall below the thermochromic activation temperature and above normal room temperature. In operation, it is understood that as this device is heated by an application of current to the silver buss bars, the initial liquid crystal material changes through the liquid crystal color spectrum from clear to reveal a first message. On further heating, this liquid crystal color message disappears and the next liquid crystal message appears. On further continued heating, the second liquid crystal message disappears and the black thermochromic layer then clears and becomes "transparent", revealing the silk-screened underlying message. Upon cooling the device, these messages appear in a reverse sequence. Continuous cycling of this device causes the messages to repeat forward and reverse for each temperature cycle.
This display device has several significant drawbacks. The heater used for this device is based upon a generally thick film (0.25 to 0.50 mil) printed carbon loaded ink heater. This printed carbon heater does not provide a suitable heater for a thermochromic display system, since the temperature across its surface is not sufficiently uniform. A printed carbon heater is formed from a polymeric binder with flakes of carbon therein, and this polymeric based ink changes resistance with time, curing and humidity during the printing process. It is very difficult to obtain a fixed uniform desired resistance value across the heater. In addition, the use of silver buss bars printed on at least two edges of this heater makes the formation of a heater an expensive process and generally requires a high-temperature cure. The liquid crystal messages, while colorful, only refract or scatter 13 to 15 percent of the ambient light and hence, are not very bright. Furthermore, passive clear spacer or barrier layers between liquid crystal messages are used to prevent contamination and add further complexity and cost to the overall device.
There is thus no known thermochromic based visual display system which provides a display with high clarity at all temperatures, fast operating times and uniform image transitions for all images of the display. In addition, there is no known display system which provides a simple, relatively inexpensive and highly efficient (both in energy usage and in visibility) thermochromic display system. Because such display systems operate on the basis of heating the display, a suitable, highly repeatable and uniform heater is necessary. In addition, it may be desirable for such a display to be suitably sheltered from ambient environmental conditions, so that changes in the ambient environment do not alter the operation or diminish the desired effect of the thermochromic based-display system.